In the past, a large number of studies on pyrimidine derivatives have been made, an immense number of compounds have been synthesized, and numerous compounds having characteristic physiological activities have been found in the fields of agriculture and medicine. However, it is not yet known that a compound having a thiocyano group introduced into the pyrimidine nucleus has been put to practical use. Very few pyrimidine derivatives having a thiocyano group have been synthesized and little is known about their biological activities. Only one instance of research on thiocyanopyrimidine derivatives and their in vitro antimicrobial activities is reported in the Journal of the Pharmacological Society of Japan, 83, 1086 (1963). This report describes 15 thiocyanopyrimidine derivatives and their antimicrobial activities, and states that the most active compound is 2(or 4)-chloro-6-methyl-4(or 2)-thiocyanopyrimidine and the antimicrobial activities of thiocyanopyrimidine derivatives having a substituent at the 5-position tend to be lower. Nothing is known about thiocyanopyrimidine derivatives having a halogen atom at the 5-position.
The present inventors have already applied for a patent on an invention concerning thiocyanopyrimidine derivatives (see Japanese Patent Laid-Open No. 193970/'85). More specifically, this invention relates to 2,4-dihalogeno-5-alkylthio-6-thiocyanopyrimidine derivatives which are useful as agricultural and horticultural fungicides, and these compounds have a fairly powerful effect. However, they are disadvantageous in that their effect does not last long enough. Meanwhile, plant pathogens have a very great influence on agricultural and horticultural production. In recent years, the availability of a variety of fungicides for diverse plant diseases has enabled a certain degree of stable production, but much yet remains to be improved.
For example, in order to control late blight, caused by algal fungi (of the class Phycomycetes), and downy mildew of various crop plants, currently available fungicides must be used in very large amounts. Moreover, their effect tends to be influenced by the time of application, the weather, and the like, and cannot be regarded as stable. Furthermore, in the case of grey mold (caused by Botrytis cinerea), stem not (Sclerotinia Sclerotiorum) and like diseases of various crop plants, the development of resistance has made benzimidazole type or dicarboxylic acid imide type fungicides practically ineffective.